Just two years ago, Republicans seemed likely to have a chance at 60 Senate seats following the 2014 elections. But things certainly changed after Democrats won 25 of the 33 seats up this year.

Instead, GOP strategists need to worry about whether Democrats will be in the position to win a filibuster-proof supermajority in 2016.

Control of the U.S. Senate is not merely about arithmetic. It is also about timing.

With one-third of the Senate up for election every two years, the key to holding 60 Senate seats — the holy grail of any Senate majority leader — is to have at least one very large Senate class.

The Democratic supermajorities in the Senate throughout much of the 1960s relied on a blowout win in 1958, when the party won 24 of that Senate class’s 32 seats. (Democrats actually did better than that on Election Day, winning another four special elections in seats not normally up in the cycle, including two in the newest state, Alaska.)

That class remained largely intact six years later, when it grew to 26 Democrats and just seven Republicans during the Johnson landslide of 1964. (A Hawaii seat had been added to the class.)

The last time one party had 60 seats in the Senate was from April 2009 to February 2010, after Pennsylvania’s Arlen Specter switched from the GOP to the Democratic Party but before Scott P. Brown, R-Mass., was sworn in. Connecticut’s Joseph I. Lieberman and Vermont’s Bernard Sanders were elected as independents but caucused with the Democrats.

The key to that supermajority was the Senate class elected in 2006, which included 24 Democrats (including Sanders) and only nine Republicans. (Two special elections held in 2008 for the class of 2006, in Mississippi and Wyoming, did not change the partisan makeup of that class.)

The two other classes that made up the Democratic supermajority in 2009 included the class of 2008, with its substantial Democratic majority (20 Democrats, 13 Republicans) and the class of 2004, which — even after Specter’s switch — actually had more Republican senators (18) than Democrats (16).

After winning 24 seats in the 2010 midterms, Republican Senate operatives surely were entitled to dream about winning 36 of the next 66 Senate seats up in 2012 and 2014.

After all, both classes were top-heavy with Democrats — 43 Democrats compared with only 23 Republicans — giving GOP strategists plenty of seats to shoot at and relatively few to defend.

Getting to 60 seats is always a struggle, but the Republicans’ huge 2010 Senate class — like the Democrats’ big classes of 2006 and 1958 — automatically put the goal of a filibuster-proof supermajority on the table.

So the Democrats’ strong showing earlier this month not only denied Republicans the Senate majority they sought in 2013 and obliterated any chance that Republicans could win a supermajority in the Senate anytime soon, it now gives Democrats a class of 25 senators and the opportunity to make a run at 60 seats in 2016.

The best news for Republicans is that Democrats have 20 Senate seats up next time, while the GOP has only 13 at risk. That should make for plenty of Republican opportunities — especially since Democrats will be defending seats in West Virginia, Arkansas, Louisiana, South Dakota and Alaska.

But Republicans had great opportunities going in 2012 and lost two seats. And, of course, Republicans have proved their ability to snatch defeat from the jaws of victory in Senate races.

Given their large class this year, Democrats need only hold their own in 2014 and then win just 15 of 34 Senate seats two years later to reach their 60-seat goal. Even if they experience modest losses in 2014, which seems likely given the seats up and the challenges faced by a president’s party in second midterms, they could still be within range of 60 seats in the 2016 elections.

Democrats will then have an opportunity to win back Senate seats that they lost because of the 2010 anti-Obama GOP wave. Their 2016 targets surely will include Republican Senate seats in Illinois, Wisconsin and Pennsylvania, as well as seats in Iowa, Florida, New Hampshire, Ohio and even North Carolina. All of those states, except for North Carolina, went for Barack Obama twice.

Obviously, both parties have a long way to go before 2016. But it’s not too early to note that the Democrats’ large class of 2012, which includes at least two solid Republican seats lost by inept GOP nominees, gives Senate Majority Leader Harry Reid, D-Nev., plenty of reason to search for the Senate’s holy grail over the next four years.